1. Field of the invention
The present invention relates to a heat treatment apparatus for heat-treating a workpiece disposed in a process tube.
2. Description of the Related Art
Various heat treatment apparatuses that perform oxidizing treatment, diffusing treatment, CVD treatment, and the like have been employed for fabricating semiconductor wafers. In conventional heat treatment apparatuses, a process tube is disposed in an upright type treatment furnace. A heater is disposed around the process tube. Wafers, which are workpieces, are placed in a wafer boat in such a manner that they are horizontally aligned therein. The wafer boat is placed in the process tube and heat treated in a process gas atmosphere at a high temperature.
In the heat treatment apparatuses, the temperature distribution of each wafer finally becomes equal. However, when the wafer boat is placed in the process tube, since the wafer boat is surrounded by the heater, the temperature at the peripheral portion of each water is higher than the temperature at the center portion thereof. Therefore, as the diameter of wafers becomes large, the temperature difference between the peripheral portion and the center portion becomes large (for example, .+-.50.degree. C.). Thus, skewing, crystal defect, or the like take place in the wafers.
In addition to the equal temperature distribution, to keep the film forming condition on the surface of each workpiece equal, the contacting conditions of the workpiece and the process gas should be improved.
When a process gas is supplied to a workpiece, if the process gas flows along the surface of the workpiece, if the gas contacting condition on the upstream side of the process gas may differ from the gas contacting condition on the other sides. In other words, the concentration of the process gas that contacts the peripheral portion of the workpiece on the upstream side of the process gas is relatively higher than the concentration of the process gas that contacts the center portion of the workpiece on the downstream side of the process gas. When the process gas contacts the center portion of the workpiece, the stream of the process gas is agitated and thereby an eddy current or a turbulence may take place. Thus, the concentrations of the process gas differs between the center portion of the workpiece and the peripheral portion thereof. As a result, since the concentration of the process gas that contacts the center portion is low, the thickness of the film formed at the center portion differs from the thickness of the film formed at the peripheral portion. The occurrences of the eddy current and turbulence of the process gas relate to critical Reynolds number R=Ud/.upsilon. (where U is the flow velocity, d is the diameter of the flow path, and n is the coefficient of kinematic viscosity).